Wednesday, 25 February 2015

Was Arabia once a lush paradise?...by Dr Ash Parton

Ash Parton

Ash Parton is on a mission to understand the relationship between environmental change in Arabia and the demography of early human populations. His PhD research, undertaken in collaboration with the BGS, has provided a unique climate record for southeast Arabia over the past ~160,000 years. This work has helped expand our understanding of early human dispersals out of Africa. ﻿If you saw the recent BBC article "Arabia was once a lush paradise of grass and woodlands" you'll already be aware of his latest results, published in the prestigious academic journal GEOLOGY. To guide us through the research in full here's Ash, lead author of the paper and a member of the Palaeodeserts Project at the University of Oxford, and some of his amazing fieldwork photos...I began my PhD at Oxford Brookes University under the supervision of Professor Adrian Parker with the rather ambitious aim of reconstructing the evolution of the Indian Ocean Monsoon system throughout the Pleistocene.

The Arabian Peninsula is a key region for research concerning both human evolution and climate studies, and so my initial idea was to develop a long transect of palaeoclimatic records running the entire length of the peninsula. This, Adrian informed me, was far beyond the scope of a self-funded PhD! Indeed, it is beyond the scope of most large, well-funded projects. Instead, he suggested that we attempt to investigate the evolution of the Arabian climate through more realistic means. It was agreed that I should accompany him on his field work to Arabia, which at the time involved working with a German archaeological team who were excavating a rockshelter site at Jebel Faya in the UAE. This site later contained evidence for the earliest human dispersal out of Africa, and while its actual age was unknown at the time, it was clearly a region for which a palaeoenvironmental framework was much needed to underpin how people were able to survive and move through this landscape.

It was during this trip that the central premise for my PhD became clear. This question has stayed with me since and still remains to be fully answered;

“how has the changing environment of this vast desert landscape shaped the fortunes of our earliest ancestors?”

From the moment I first set foot among the sands of the Rub’ al-Khali, I was amazed to think how dramatically different it had once been. A number of palaeoclimatic records had already shown that while now arid/hyper-arid, at times the landscape of Arabia was littered with rivers, lakes and extensive grasslands. It was clear that such changes would have had a profound effect on any early populations, and that by better understanding the timing and nature of these changes we might better understand our own demographic trajectory.

Previous studies had suggested that the occurrence of humid periods in Arabia were predominantly driven by global ice volume changes. In particular, the development of interglacial conditions every ~100,000 years was seen as the principal driver of increased rainfall across the peninsula. As the major ice sheets contracted, monsoonal rainfall shifted much further north, bringing with them large volumes of summer rainfall and transforming the arid environment. Conversely, during global glacial periods these systems were pushed further south and Arabia returned to the harsh, dry conditions we see today. As I began to explore the full range of climatic records, however, it became clear that some records told a rather different story. Marine records seemed to show that the monsoon experienced these phases of intensification and northward displacement every ~23,000, in line with periods of maximum solar radiation. Why then, I wondered, do we not see the same evidence in the terrestrial records? Confirming the presence of additional wet phases in Arabia then became a key aim of my work, as the occurrence of such periods would have provided further ‘windows’ for human populations to expand out of Africa.

Fig. 1 from Parton et al.: Map showing location of the study site and
extent of bajada system in southeast Arabia, including other identified
sections of the Al Ain fan (UAE—United Arab Emirates).
Image from GEOLOGY press release (c) GSA

In order to answer this question a greater range of climatic records was required. Previous climatic reconstructions had relied predominantly on speleothems, which require a lot of rainfall to form and as such, a significant range of rainfall remained undetectable from these records. In the first instance I had managed to locate an ancient lake deposit near the rockshelter site at Jebel Faya. Initial optically stimulated luminescence (OSL) dates indicated that the lake formed ~55-60,000 years ago – nicely in the middle of a glacial period – suggesting that the patterns of rainfall I had seen in the marine records did have an expression on the land. This was just one relatively small palaeolake site, however, and I needed more proof that glacial-age humid phases were more than just climatic ‘blips’. After a brief worrying period in which no further sites were found, I was fortunate enough to begin a collaboration with the BGS. Adrian had started working with a BGS team who were mapping the geology of the UAE and in 2008 one of the team, Dr Andy Farrant, introduced me to the site that would vastly improve our understanding of the Arabian palaeoclimate. Situated near the town of Al Ain, recent quarrying had exposed a deep ~42 m sequence of alluvial fan deposits at the site of Al Sibetah. These ancient river channel and soil sediments comprised the biggest record of major climate changes from anywhere in the peninsula, and provided evidence of multiple periods of increased rainfall.

Over the following year, successful collaborations with Andy Farrant and with Professor Melanie Leng of the BGS Isotope Geosciences Laboratory, and Dr Matt Telfer then from the University of Oxford, led to the development of a unique record of climate change for southeast Arabia that has had important implications for our understanding of early human demography. OSL dating of the sequence indicated that the activation of rivers within a vast bajada along the western Hajar and Oman Mountains had occurred approximately every 23,000 years since at least ~160,000 years ago, confirming that monsoon incursions into Arabia did in fact occur in line with insolation maxima. During these periods, expansive river systems surrounded by verdant savannah grasslands and trees, connected the mountains and the coast, potentially acting as vast green corridors through which early human populations could move. Importantly, the findings suggest that there were numerous windows for the dispersal of human populations out of Africa, and that demographic mobility was not restricted to interglacial periods every ~100,000 years.

Issues concerning the spatial and temporal variability of the Arabian climate continue to be the main focus of my research, and that of the Palaeodeserts Project. The findings from the Al Sibetah fan demonstrated that hidden beneath the sand seas of Arabia lies evidence for a complex and incredible climatic history. Despite many years of study, we are still just scratching the surface, quite literally, when it comes to understanding the evolution of the Arabian environment. And yet, the unfolding archaeological record continues to prove that the region played a critical role in the development of our species.

The findings from the Al Sibetah alluvial fan were recently published in the journal GEOLOGY, and reported by media outlets such as the BBC, Science Daily and the Daily Mail. Its been a very exciting week! By Ash Parton
PhD supervision at BGS by Dr Andy Farrant (fieldwork and sedimentology) and Prof Melanie Leng (stable isotope geochemistry).